Lord Armstrong, who was the country’s most senior civil servant during the Thatcher era, has been branded ‘cavalier’ in his apparent defense of MI5 inaction over Westminster VIP pedophile allegations.

His comments were in response to claims by campaigners that a letter from MI5 to the ex-civil servant found among a batch of Cabinet Office documents suggested there had been a child abuse cover-up involving ministers.

In 1986, Lord Armstrong was warned by the domestic spy agency that a particular MP, who has not yet been named, had a “penchant for young boys.”

MI5’s then-Director General Sir Anthony Duff said “at the present stage … the risks of political embarrassment to the government is rather greater than the security danger.”

No action was taken at the time and on Thursday Armstrong defended the decision, saying that “shadows of a rumor” had not merited further investigation.

He defended the security services response to the allegations.“I think they were right to report the rumor, they were right to make what inquiries they could and they were right to come to the conclusion they did. I think if there was evidence it would have been properly examined at the time.”

Armstrong also denied that public figures had been deliberately protected.

“I don’t think this is a matter of important people being protected. You can’t pursue inquiries unless you have evidence on which you can base the enquiry. A shadow of a rumor is not enough.”

“It’s a criminal matter and it beggars belief that someone at the heart of government should show such a lack of interest in protecting children,” he said.

“We know that young boys were raped by powerful people causing untold damage and it looks like the only priority for those running the country at the time was to protect the reputation of the government at all costs.”